Separate async and blocking examples

[zpg6]

When async becomes available from embedded-sdmmc-rs:

• Duplicate bin/main.rs into bin/blocking.rs and bin/async.rs
• Update bin/async.rs to use the async driver
• Update bin/async.rs to do the sd card R/W in Embassy tasks to better demonstrate async
• Remove embassy from bin/blocking.rs, just use an esp-hal entrypoint

The latest PR to tackle async looks promising:
rust-embedded-community/embedded-sdmmc-rs#176